The invention is in the field of vacuum tubes, and more particularly relates to a coated anode designed to reduce out-gassing, plasma formation, and secondary electron production.
Every vacuum electronics device, ranging from radio frequency tubes to microwaves tubes, must have some region in which the cathode emitted electrons impact after participating in the desired interactions. Generally these structures consist of stainless steel, oxygen free high conductivity (OFHC) copper or some other metal. Occasionally the metal is coated with an insulating material such as titanium nitride. Metals are generally the optimum structures due to the good electrical and thermal conductivity as well as the superior vacuum performance.
One major drawback with these materials is the production of secondary electrons, plasmas, and neutral gasses upon electron impact. Neutral gasses contribute to raising the pressure in the tube, reducing the vacuum. Plasmas not only increase the pressure but also cause the tube to short electrically, limiting the duration of microwave or radio frequency output. Plasmas can also cause damage other components, e.g., the cathode or other metallic structures. Secondary electrons are electrons produced by the impact of the primary electron beam. A single primary electron can produce several or as many as hundreds of secondary electrons. These secondary electrons then cause the formation of plasmas and result in further out-gassing from the metal anode or collector.
These problems are amplified when the collector is biased to allow energy recovery from the primary electron beam. Here, the secondary electrons can easily be re-accelerated back into the collector, causing a cascading process producing more secondary electrons. One method to reduce this effect is to coat the anode/collector with a carbon film. The carbon reduces, but does not eliminate the effects discussed above.
Accordingly, there is a need for an anode/collector that can significantly reduce the production of secondary electrons, plasma formation, and out-gassing of neutral gases.
In a preferred embodiment, the anode/collector surface of a vacuum tube is coated with a carbon nanotube material having the longitudinal axis of at least a portion of the nanotubes running parallel to the surface. The anode/collector surface initially is comprised of carbon or a metal surface coated with a thin film of carbon. It is then coated with a carbonizable resin. The final coating can be a carbon nanotube felt-like material that is pyro-bonded to the anode surface, or nanotubes can be deposited on the anode by chemical vapor deposition or by evaporation and then pyro-bonded.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.